$($a$)$ There are two speakers and one microphone located with positions shown in the following figure. The sound sources are not correlated with each other. It is assumed that two sources can uniformly radiate sound in all directions in air. The sound powers of speakers $1$ and $2$ are $6$ W and $8$ W $,$ respectively. Determine the resultant sound pressure level at the microphone when the sound sources are in operation simultaneously. Assume that the microphone is directly affected by these two speakers, and there is no reflective sound effect. Air density is $1\mathrm{.}29k\frac{g}{m^3},$ and sound speed is $344\frac{m}{s}.$

$(5$ marks$)$

Figure $1.$ Configuration of Speakers and Microphone $($not to scale$)$